Grapple skidders are forestry work machines used to haul logs, typically over rugged terrain. A skidder includes a grapple assembly located at one end of the skidder to pick up, haul, and later release a load of logs. The grapple assembly generally includes a grapple head that is actuated through various positions, and a hydraulic motor for rotating the grapple head. The hydraulic motor is utilized to position the grapple head at a proper orientation with respect to a load of logs to be hauled. This allows an operator of the skidder to approach the logs from any direction in order to grab the logs and secure them in a grasping configuration of the grapple head. Hauling is typically accomplished by grasping a load of logs at one end and dragging the logs behind the skidder.
When the grapple head is in a grasping configuration, i.e., logs are being carried or dragged by the grapple head, a tremendous torque may act on the grapple assembly when the skidder turns and the logs rotate relative to the machine or resist turning of the skidder. Specifically, when turning the skidder, the logs lag in the turn thereby twisting the grapple head and rotating the hydraulic motor. This force creates a torque against the motor and forces it to act as a pump. This can result in cavitation which can lead to reduced motor life and other stresses on hydraulic circuitry.
To account for this, pressure relief systems have been devised to flush or release pressure within a hydraulic system. For example, U.S. Pat. No. 5,018,935 includes two spring biased pressure relief valves. A first valve is hydraulically coupled to a first supply/return line of a hydraulic motor and a second valve is hydraulically coupled to a second supply/return line of the hydraulic motor. When pressure becomes too high in either the first or the second supply/return line, the valve coupled to the supply/return line will be hydraulically actuated to open a bypass line to relieve pressure to the hydraulic motor. Since an adequate amount of pressure will be required to operate the hydraulic motor, this reference only provides relief as described above when pressure within one of the supply/return lines reaches an overpressurized state. In fact, the threshold at which the relief valves are set to actuate may be so high, to allow for normal operation of the hydraulic motor, that cavitation could occur before either of the relief valves are actuated.
The present disclosure is directed to one or more of the problems set forth above.